Playing surfaces across all levels of athletic competition have changed considerably over the past half century. Advancements and changes were made to improve field performance and provide financial benefit via increased field usability across multiple environments and activities. Artificial turf playing surfaces are now common at all levels of competition from professional to community-based youth programs. This article seeks to evaluate current evidence regarding the safety profile of various athletic surfaces across a variety of sports.
Review of Playing Surfaces
Over the past half century, advancements in engineering and technology have spurred the progression of various types of athletic play surfaces across multiple sports. These surfaces range from natural grass (NG) to multiple generations of artificial turf, as well as different types of playing courts. A brief review of these surface types is presented here.
The advantages of NG for athletic competition include a more natural game playing experience with regards to cleat-surface interaction, ball roll, and speed of play (1). The major downfall of NG surfaces is the significant resources (including time, manpower, and water) required to maintain a consistent, clean, divot-free playing surface. Professional and high-level collegiate athletes who train and compete on NG, do so on well-groomed grass fields. At lower levels of competition, field-sharing by multiple teams leads to higher turf wear and often produces an uneven playing surface. Therefore, it is difficult to maintain NG surface quality in multisport environments. Finally, NG is affected heavily by environmental factors including varying temperatures, rainfall, droughts, and winter weather which inhibit its year-round athletic use. These factors have led to the development of artificial turf over time.
Artificial Turf (AT) was first developed in the late 1960s as an alternative to NG for athletic competition. AstroTurf, the original AT, consisted of short fibers of dense nylon carpet installed over a compacted soil base or elastomeric foam pad. While cost effective, the surface led to higher injury rates because of its firmness and very high coefficient of friction (2). Subsequent generations of AT have evolved in an attempt to create a playing surface with more natural feel while still maintaining long-term cost benefits.
Generation 3 AT utilized longer synthetic fibers with infill pellets made of rubber or granular material to produce a turf that closely resemble play and force absorption of NG. Since the introduction of Generation 3 AT, various types of infill have been developed. These infills improve resistance to weathering, sustain field durability, and increase cushioning/shock absorption for athletes. Newer infills also provide adequate coverage of inseams on the underlying carpet, helping to prevent injuries caused by shoe/cleat catching that were seen with earlier generations of artificial playing surfaces.
AT has numerous benefits, including long-term durability, ease of maintenance, and cost benefit. The durability of AT allows for multi-use potential for games and entertainment (such as concerts) across a variety of environmental conditions without a deleterious effect on the playing surface (3). Furthermore, these new-generation ATs are resilient and able to withstand high volumes of play while still maintaining a consistent playing surface game after game (2). While maintenance of AT is less costly than NG, it still requires regular upkeep. Brooming with a nylon bristled brush to help raise matted fibers, periodic replacement of lost infill, and occasional removal of paint and debris buildup, are necessary maintenance. Despite the benefits of AT, concerns do exist about the overall safety profile of AT, including both orthopedic and general medicine concerns. We will further discuss these concerns below.
Tennis courts for professional and recreational use vary greatly in composition. The sport was initially played only on NG composed of seeded turf on a soil base. Many popular competitions, such as Wimbledon, continue to be played on grass today. However, great care is necessary to ensure grass courts are well maintained for competitive use, and these courts are heavily affected by the local environment (4).
Clay courts became popular for use in the 1950s and are generally composed of layers of crushed stone topped with fine, gritty clay (5). Clay courts are common in continental Europe and Latin America. Notably, the French Open is played on clay. Two types of clay court exist; red clay, from crushed brick, and green clay, from crushed metabasalt. Clay surface has a very low friction coefficient with the player, allowing significant sliding on the court. In contrast, the friction coefficient with the ball is higher causing ball speeds to be generally slower (4).
Lastly, tennis also is commonly played on acrylic courts composed of underlying asphalt or concrete coated with rubber for shock absorption. This court surface is increasingly popular due to lesser need of maintenance and the associated cost-benefit. However, acrylic courts are stiffer and have the highest player-surface friction coefficient, suggesting an increased risk of overall injury to lower extremities compared with other court surfaces (4).
Indoor sport surfaces
With increased popularity of year-round athletic participation, it is important to consider the various aspects of indoor playing surfaces. Various sports promote indoor play: basketball, volleyball, handball, and indoor track and field. The various surfaces include synthetic courts, wood courts, smooth concrete, asphalt court surfaces, and rubberized tracks. Currently, cost is a large determining factor for surface choice as evidence is limited to suggest recommendations on safety and risk estimation. In the absence of literature discussing injury rates between various indoor sport surfaces, we will defer further discussion of surface composition and maintenance.
Although limited, there is literature that describes higher bone mineral density in young females playing basketball and handball on synthetic and smooth concrete surfaces compared with those playing on parquet and synthetic courts (6). Unfortunately, there have not been studies to evaluate how these different forces translate to risk of acute or chronic lower extremity injuries in basketball players and other indoor court sport participants. For indoor running tracks, various surfaces (concrete, asphalt, synthetic rubber) alter vertical acceleration and shock absorption, thus, it would be expected that injury rates would be higher on firmer surfaces; however, no studies have evaluated this theory.
An analysis of athletic play surfaces would be incomplete without consideration of their associated orthopedic injuries. A review of currently available evidence has identified various associations of such injuries with differing sports and play surfaces.
Since the allowance of artificial playing surfaces by the Fédération Internationale de Football Association (FIFA) in the early 2000s, artificial playing surfaces have been viewed with reservation by elite level players due to concerns of higher injury risk. Fortunately, this belief has not been supported by studies across the spectrum of skills levels. In youth/amateur soccer, two Norwegian studies demonstrated no difference in overall injury rates between the NG and AT surfaces (7,8). Soligard et al. (7) found a reduced risk of ankle injuries but increased low back pain and shoulder/clavicle injuries in games played on AT compared with NG. In Japan, playing soccer on AT greater than 80% of a player's season was found to be a risk factor for fifth metatarsal fractures (9).
Within National Collegiate Athletic Association (NCAA) soccer, the trends found in European youth/amateur soccer held true. Two prospective cohort studies evaluated FieldTurf® (specifically Generation 3 AT) versus NG, and found lower overall injury rates in games on AT compared with NG in both men's and women's soccer (10,11). Additionally, no difference was seen in lower extremity injuries between the two surfaces. Older artificial playing surfaces (older than 8 years) were found to be safer than similarly-aged NG surfaces (10). Work with the NCAA Injury Surveillance System for men's and women's soccer by Fuller et al. (12,13) also found no difference in overall injury rates in both competition and training sessions. Looking at lower-extremity injuries specifically, no difference in injury rates were seen in competition between the two surfaces; in training sessions, however, differences did emerge. Male athletes suffered more foot/ankle injuries on AT while female athletes experienced more lower-extremity injuries on NG (12,13). In a recent study, Howard et al. (14) compared and found no overall difference in competition injury rates between AT and NG. Injury rates during training sessions were higher on NG in this 2020 study.
At the highest levels of competition, artificial surfaces continue to demonstrate safety in comparison to NG surfaces. Three studies in top tier European soccer found no difference in overall acute injury rates in both male and female professional soccer players (15–17). Ekstrand et al. (15) did find a higher rate of ankle injuries in male professional players on AT in leagues across Northern Europe. In the United States, Calloway et al. (18) examined and found no difference in acute injury rates between the two surfaces in Major League Soccer (MLS) competition. Additionally, MLS players had no difference in knee injuries between the two surfaces.
With the increase in the use of artificial playing surfaces, the study of playing surface-based injury rates within American football has continued to expand; however, the results have been conflicting. Within NCAA football, Dragoo et al. (19) found higher anterior cruciate ligament (ACL) injury rates on AT. Loughran et al. (20) identified that in the NCAA, posterior cruciate ligament injuries were more common on AT regardless of competition level, while ACL injuries were only higher on AT in divisions 2 and 3 competitions. In this study, there was no difference in injury rates in the practice environment. In contrast, Meyer's study (21) saw lower overall ligament and muscle injuries on FieldTurf® compared with NG surfaces but found no difference in knee injury rates between the two surfaces.
In professional football studies among National Football League (NFL) players, most of the data point to a higher risk of injury on artificial playing surfaces. Hershman et al. (22) found increased overall knee and ankle injuries on AT. While ACL injury rates were found to be significantly higher, no difference was found in MCL or lateral ankle injuries. Mack et al. (23) found a 16% increase in overall lower extremity injury risk. Additionally, noncontact injuries in the knee and ankle were found to be higher on artificial surfaces. Compared with sheer failure as an inherent safety mechanism of NG, the inability of synthetic surfaces to release a cleat under a potentially injurious force has been postulated as a mechanism of the increased lower extremity injuries seen on AT in this population (24). Significant research has been pursued on this subject, especially for development of footwear to aid in prevention of these injuries which is beyond the scope of this article.
In contrast to the findings above, Lawrence et al. (25) found no difference in acute knee or ankle injuries in NFL games on NG vs AT. Shoulder injuries were actually higher during games on NG in this study. Finally, playing surface was found to not be a risk factor in Achilles tendon injuries in the NFL player population (26).
Unfortunately, little is known about the effects of playing surface on injuries at lower levels of competition, including high school and other scholastic competitions. This is an area prime for future research.
AT playing surfaces are becoming more common at the NCAA and Major League Baseball competition level. Despite this increase in popularity, we were unable to find any studies comparing injury rates in baseball between different playing surfaces. These authors presume that injury risk/patterns would follow soccer more closely than American football; however, this needs to be fully assessed in future research.
Similar to findings in soccer, studies in European and Asian elite level rugby union have found no significant difference in the overall acute injury rates between AT and NG in rugby union (27–30). Fuller et al. (27) compared acute injury severity during competitions and found no difference between the two surfaces. Their findings did show a trend of increased ACL injuries on AT, but this did not meet statistical significance. Ranson et al. (28) found higher rates of concussion and chest injuries on NG, while thigh hematomas and foot injuries were more common on AT. In their study in English Premiership rugby union competition, Williams et al. (29) identified higher abrasion rates on AT but this did not lead to an increase in missed time or other complications. Additionally, visiting players who normally played on NG, experienced prolonged muscle soreness after games on AT. In addition to the acute injury differences found above, Lanzetti et al. (30) found that AT increased the risk of overuse injuries in Italian rugby players.
Tennis has a variety of surfaces available for play. Due to variations in the friction coefficient, ball speed, and shock absorption, each court surface has an interesting injury risk to consider.
Tennis players on hard courts suffer more injuries of lower extremity joints and ligaments rather than soft tissue strains and trunk injuries (31). A valuable consideration for the increased risk of these specific injuries is whether hard courts place higher force on athletes' joints and affect movement patterns. Higher impact force on the lower extremity during play and an increased coefficient of friction between the athlete and the court, predisposes athletes to lower-extremity injuries (5). To address this further, a biomechanical study of playing surface properties described that with a lower coefficient of friction between footwear and a surface, the sliding movements require greater muscular control, increasing the risk of fatigue and potentially injury (32). This can be clinically applicable to assist in developing differential diagnoses based on court type, in clinically complex cases.
This is further supported by the lack of statistically significant difference in overall injury rates among experienced club tennis athletes on different court surfaces. However, athletes playing on hard court had an increased proportion of injuries to the lower extremities than those playing on grass and clay (33,34). Physicians, athletic trainers, and all others involved with the care of athletes should understand that athletes playing on hard court surfaces likely have higher risk of lower extremity injuries.
General Medical Considerations
In addition to air temperature, infrared heating of the playing surface contributes to the multivariate risks of athlete Exertional Heat Illness (EHI). A 2017 study by Pryor et al. (35) demonstrated that using the national Wet-Bulb Globe Temperature (WBGT) underestimates the temperature in arenas/stadiums with artificial playing surfaces. The WBGT should be measured onsite to appropriately estimate EHI risk. In addition to EHI, in a 2004 prospective study, Meyer and Barnhill (36) found a significantly higher incidence of total injuries on FieldTurf® compared with NG on days where temperature was over 70°F, but not on cold days (temperature <69°F). The goal to reduce the risk of EHI and injury has initiated technological advances such as cool-climate surface products and trials of irrigation to decrease the heat retention of AT. In 2014, Petrass et al. (37) found that although both Generation 3 AT and cool-climate products showed higher temperatures than NG, the cool-climate surface was significantly cooler. At the time of this publication, the authors are not aware of any studies evaluating incidence of EHI on AT versus NG fields. It is valuable to continue to evaluate the multiple variables, including air and surface temperatures, affecting EHI and other injuries to better understand the complex risk estimation. In the interim, physicians, coaches, and organizations that support athletes in areas where EHI is a significant concern should continue to consider and recommend appropriate risk mitigation strategies such as rest/work cycles, appropriate hydration, intermittent cooling, alternate training locations/facilities and cross-training activities.
Abrasion and infection risk
In addition to the individual risk of an athlete whose injury is related to contact with the playing surface, there is risk of communal spread of potential infection across a team. Breaks in the skin caused by turf burns have been correlated to bacterial infection, and many case studies have looked to describe these outbreaks, generally recommending that the best way to reduce the risk of infection is to protect against skin injury (38).
Common and concerning infections are caused by bacteria resistant to empiric antibiotic therapy, such as methicillin-resistant Staphylococcus aureus (MRSA). MRSA was evaluated by Keller et al. (39) for survivability on various infill types including sand, which was measured to be 93% survivability of MRSA, cork, 90%, and crumb rubber, 71%, compared with 98% survivability of the control, pointing toward levels of substance toxicity to MRSA. McNitt et al. (40) looked specifically at survivability of S. aureus on AT versus NG and found that the bacterial populations dropped significantly and over shorter periods of time, in outdoor studies compared with indoor and were dramatically reduced on AT compared with NG. Complicating the application of this result is the players perception that AT is more abrasive. A systematic review by Twomey et al. (41) evaluated surface abrasiveness showing no significant disadvantage of the AT, although this study was limited by inconsistencies in injury definition and limited details of the surfaces. There is a paucity of evidence comparing infection rates to various athletic play surfaces.
Toxicity related to exposure
Human health risk related to inhalation, skin absorption, and ingestion of AT infill has been of ongoing concern. The conflicting goals of athletes, teams, parents, public health policy makers, manufacturers, sporting organizations, and facility managers requires careful consideration of economic, environmental, and overall health priorities. Crumb rubber (tire crumb) is the most commonly used infill of generation 3 and later AT. Made of vehicle tires, it is potentially carcinogenic and toxic. Alternate infills have been developed to improve the characteristics of AT. Concerning compounds include volatile organic compounds (VOCs), polyaromatic hydrocarbons (PAHs), lead and other metals, fungi, allergens, and pulmonary dusts such as silica. Previous studies performed by multiple health departments including the Netherlands National Institute for Health and Environment and the Connecticut Department of Public Health evaluated levels of various concerning compounds, determining them to be below human health risk levels and comparable to community exposure (42,43). In 2020, Massey et al. (44) compared chemical concerns of various AT infills including tire crumb, ethylene propylene diene terpolymer, athletic shoe infill materials, thermoplastic elastomer, acrylic-coated sand, and mineral- or plant-based infills. This study found that acrylic-coated sand and mineral- and plant-based infills had lowest or absent levels of VOC, PAH, and metal substances, but are most likely to have fungi, allergens, biologically-active, and pulmonary fibrogenic dust. Tire crumb, the most common infill type, on the other hand, had the highest level of PAHs and lead (44). Further studies are needed to evaluate modern AT exposure risk, clinical significance, and epidemiology.
With growing recognition and concern for long-term risk related to head injury, the risk of concussions on AT compared with NG is an important consideration for physicians, coaches, and organizations. It is complicated by the above components that potentially alter an athlete's play on a particular surface as well as the player's perceptions. In 2007, Fuller et al. (12) confirmed the results of Meyer from 2004 describing that head to surface concussion occurred less frequently on AT than on NG in both male and female soccer players (36). Furthermore, a systematic review done by O'Leary et al. (45) suggests that in competitive contact sports, concussion rate is lower on AT than on NG.
As technology advances to allow materials that are more environmentally friendly or economical to be used for practice and competition surfaces, it is important to be aware of the growing evidence that will help guide athletes, teams, and community care and support of continued safe sport participation. Although there is no conclusively ideal surface available at this time, ongoing research is needed to continue to evaluate the safety of further advances and recommend progress in a medically conscientious direction. The components affecting play and safety are not limited to AT and infill. Rather, the environmental effects, player perceptions, microbiome, player-surface interface, and quality of maintenance all impact the forces and movement of the athlete and team. This article aimed to summarize the more recent evidence of the safety of these characteristics on the field and court.
The authors declare no conflict of interest and do not have any financial disclosures.
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